Most are no bigger than a speck of dust, yet some have the power to kill fish with their toxic slime. A few are herbivores, but the majority are predators. All have the ability to regenerate any part of their body. First documented in Europe in the early 18th century, new species are still being described at the rate of up to 50 each year.

“To the microscopic world, they’re ferocious dreaded beasts,” says University of Maine biologist Seth Tyler of his favorite invertebrates, marine turbellarians or flatworms, which he has been studying around the world for almost three decades. “They’re like vacuum cleaners going around sucking up other little animals.”

Perhaps even more remarkable is the potential of turbellarians to help answer fundamental questions about the relationships of all animals, including their origins in the primitive Earth and genetic connections to the major branches of the animal kingdom.

“They tell us how animals changed from what we know are the primitive forms, such as jellyfish, to being bilaterally symmetrical,” says Tyler, who started studying turbellarians as a graduate student at the University of North Carolina. “These animals also have a very special type of stem cell. They essentially invented the stem cell among early bilaterally symmetrical animals.”